1. Field of the Invention
The invention relates to a process for the preparation of flexible polyester urethane foams by reacting polyester polyols, having at least two hydroxyl groups in the molecule, with at least difunctional polyisocyanates in the presence of water, and the optional presence of physical blowing agents, catalysts, stabilizers, and other conventional additives.
2. Description of the Prior Art
In the preparation of flexible polyester polyurethane foams, N-alkylmorpholines or dimethylbenzylamine are generally used as catalysts. During the foaming process, these catalysts exhibit a high selectivity for the reaction of the polyisocyanate with water. This selective catalysis is required in order to obtain foams which are as open-celled and non-shrinking as possible. However, such catalysts are disadvantageous in that, on the whole, their catalytic effectiveness is weak. Therefore they have to be added to the foaming formulation in relatively large amounts. This is responsible for the fact that the foams have a strong and unpleasant odor after curing.
As a result, there have been numerous attempts to find other catalysts with less odor. For instance, U.S. Pat. No. 3,954,749 discloses the use of .beta.-aminocarbonyl compounds, such as, for example, 3-(N-morpholino)-N',N'-dialkylamides.
U.S. Pat. No. 3,925,268 discloses the use of .beta.-tert.-aminonitriles, e.g., 3-(N,N-dimethylamino)-proprionitrile, as catalysts.
Further compounds are disclosed in U.S. Pat. Nos. 3,821,131, 4,011,223, 4,012,445, 4,038,210, 4,049,931, 4,115,321 and 4,122,038.
European Patent Application No. 0 048 985 describes catalyst mixtures which consist of at least one hydroxyalkylpiperazine and one other tertiary amine.
While these catalysts do have the advantage of reduced odor, they also exhibit a reduced selectivity for the isocyanate-water reaction relative to the polyol/isocyanate reaction and therefore, lead to a closed cell foam. Moreover, .beta.-propionitriles are not absolutely physiologically safe.
The use of mixtures of catalysts for the preparation of flexible polyester urethane foams has also been proposed. Mixtures of morpholine derivatives, as selective catalysts for the isocyanate/water reaction and other catalytically active amines have been mentioned in particular. For example, the addition of small amounts of N,N'-dimethylpiperazine to alkylmorpholines is known from European Patent Application No. 0 082 597. The amount added is 0.05 to 0.15 parts by weight for every part by weight of alkylmorpholine. If the amount of N,N'-dimethylpiperazine is increased, polyester foams are obtained which have a reduced open-cell character and which, moreover, tend to shrink. A similar process is described in U.S. Pat. No. 4,326,042, in which a mixture of 5 to 15 parts by weight of N,N-dimethylpiperazine with 20 to 60 parts by weight of N-butylmorpholine and, in addition, 20 to 60 parts by weight of N-methoxypropylmorpholine is mentioned as catalyst for polyester urethane foams. A mixture of 50 to 90 weight percent of 4,4'-dimorpholinodiethyl ether and 50 to 10 weight percent of N,N'-dimethylpiperazine is recommended as catalyst for the production of flexible polyester urethane foams in German Patent No. 21 38 402. It is a common feature of all of these processes that the upper limit to the amount of N,N'-dimethylpiperazine in the catalyst mixture is only 50 weight percent. As a result, the overall amount of amine catalyst to be used remains unreasonably high. It is not possible to increase the proportion of N,N'-dimethylpiperazine in the catalyst mixture, since the polyester foams formed would have a decreased open-cell character. According to the teachings of German Auslegeschrift No. 11 68 633, it is not possible to use N,N'-dimethylpiperazine as the only catalyst, since this would lead to the formation of polyester foams with a non-uniform, coarse cell structure.
A process for the preparation of flexible polyester urethane foams is also known from German Auslegeschrift No. 12 82 949. This is carried out by reacting polyesters having an average of at least two hydroxyl groups per molecule and a hydroxyl number of about 45 to 150, with polyisocyanates in the presence of a blowing agent, polysiloxane-polyoxyalkylene block copolymers as a surface active material, and catalysts for the urethane formation reaction. Block copolymers with a molecular weight of 3,000 to 17,000 and a polysiloxane content of about 14 to 42 weight percent and an oxyethylene content of at least 75 weight percent, based on the total amount of oxyalkylene groups, are used in the polyoxyalkylene block. The polysiloxane block has siloxane groups of the general formula ##STR2## in which R is a monovalent hydrocarbon radical or a bivalent organic radical and at least one of the R substituents is a bivalent organic radical which is linked to a polyoxyalkylene block and b=1 to and including 3. The catalyst used has a higher activity than that of N-ethylmorpholine. Alternately, block copolymers with a molecular weight of about 3,000 to 7,000 and a polysiloxane content of 14 to 20 weight percent as well as an oxyethylene content of at least 75 weight percent, based on the total amount of oxyalkylene in the polyoxyalkylene block can be used. The polysiloxane block will have siloxane groups of the polysiloxane block of the above-given general formula ##STR3## which, as above, are linked to the polyoxyalkylene block, and wherein a conventional polyurethane-forming catalyst is used.
This Auslegeschrift does not, however, disclose a rule for the selection of the catalyst, since N-ethylmorpholine is a very weak catalyst and the majority of the catalysts used are more effective than N-ethylmorpholine. As highly active catalysts, compounds, such as, triethylamine, N,N,N',N'-tetramethyl-1,3-butanediamine, triethylenediamine, N,N-dimethylethanolamine, dibutyl tin dilaurate, dibutyl tin di-2-ethylhexoate, tin(II) octoate or tin(II) oleate are mentioned. It has turned out, however, that foams of inadequate quality may result, if these highly active catalysts and the copolymers mentioned are used. For example, the dimethylethanolamine catalyst leads to core discolorations, while tin octoate and triethylenediamine form closed foams.